Zinc oxide is one of the most promising materials for optoelectronic applications due to its wide band gap of 3.37 eV and large exiton binding energy of 60 meV. Zinc oxide nanostructures have wide range of potential applications also in areas such as solar cells, field emission devices, chemical and biological sensors, photocatalysts, light emitting devices, including light emitting diodes, and nano-sized lasers.
Flat zinc oxide layers (i.e., as opposed to a layer, comprising nanorods, nanoneedles, nanowires, etc structures) are widely used for electronic and optoelectronic devices, for example, as transparent electrodes in thin film solar cells where simultaneously a high transparency and a low resistivity is required, but also in thin film gas sensors, varistors, and surface acoustic-wave devices.
Flat zinc oxide layers are conventionally prepared by several technologies, including sputtering, chemical vapour deposition, sol-gel deposition, atom layer deposition, molecular beam epitaxy, and different spray pyrolysis technologies (ultrasonic spray, pneumatic spray, pressure spray). In contrast to the other deposition techniques, the advantage of spray technique is its extreme simplicity. So the capital cost and the production cost of high quality metal oxide semiconductor films are expected to be the lowest compared to all other techniques. Furthermore, this technique is also well suited for mass production systems.
Chemical spray pyrolysis is a well-known, cheap and simple deposition technique to prepare thin films of metal oxides, sulfides and tellurides, etc. for application in electronics and optoelectronics. U.S. Pat. No. 3,148,084 to Hill (Sep. 8, 1964) for a process for making conductive film describes a process of making homogeneous microcrystalline semiconductive and photoconductive films, e.g. cadmium sulphide. The method was simpler to operate, and more efficient, versatile and economical than previously known methods of forming semiconductive layers.
Spray technologies have been used for different materials and applications by Chamberlin R. R. et al (Chemical Spray Deposition for Inorganic films, J. Electrochemical Soc. 113 (1966) 86-89), Feigelson R. S. et al. (II-VI Solid Solution Films By spray Pyrolysis, J. Appl. Phys. 48 (1977) 3162-3164), Aranovich J. et al (Optical and Electrical Properties of ZnO Films Prepared by Spray Pyrolysis for Solar Cell Application, J. Vac. Sci. Technol. 16 (1979) 994-1003), Turcotte R. L. (U.S. Pat. No. 4,338,362 for Method to synthesize and Produce Thin Films by Spray Pyrolysis, issued Jul. 6, 1982), Major S. et al (Thin Solid Films, 108 (1983) 333-340, Thin Solid Films, 122 (1984) 31-43, Thin Solid Films, 125 (1985) 179-185), Ortiz S. et al (J. of Non-Crystalline Solids, 103 (1988) 9-13, Materials Chemistry and Physics, 24 (1990) 383-388), Caillaud F. et al (J. European Ceramic Society, 6 (1990) 313-316).
To prepare flat films of zinc oxide by spray usually zinc salts e.g. zinc acetate, zinc nitrate etc. can be used as precursor materials. Appropriate additives as salts of Indium, Aluminum or Terbium were added into the spray solution to make the films electrically conductive (European Patent application No 336574 to Sener for producing a layer of transparent conductive zinc oxide, priority date 6 Apr. 1988) and cobaltous or chromium acetylacetonates to accelerate the growth of the films in spray process (European Patent No 490493 to Platts for A process for depositing a layer of zinc oxide onto a substrate, date of filing 14 Nov. 1991, priority 12 Dec. 1990;U.S. Pat. No. 5,180,686 to Banerjee for Method for continuously depositing a transparent oxide material by chemical pyrolysis, issue date Jan. 19, 1993).
Zinc oxide nanopowder is also widely used, e.g., in sunscreens, paints, plastics, cosmetics because of its property to absorb ultra-violet radiation. Different methods are used to produce such powder. Spherical ZnO microcrystals could be obtained by spray pyrolysis (see, e.g., M. Andres-Verges, et al, J. Materials Science 27 (1992) 3756-3762, Kikuo Okuyama et al Chemical Engineering Science 58 (2003) 537-547, Kang, Y. C. et al Journal of Aerosol Science, 26 (1995) 1131-1138). In U.S. Pat. No. 6,036,774 to Lieber (filing date 22 Jan. 1997, issue date 14 Mar. 2000) for method of producing metal oxide nanorods describes metal oxide nanorods with diameter between 1 and 200 nm and aspect rations between 5 and 2000, produced by controlled vapour-solid growth processes in a furnace from a metal vapour source such as a mixture of a bulk metal oxide powder and carbon powder, and a low concentration of oxygen gas.
Rod-like zinc oxide nanoparticles/crystals of different size are made by deposition from solutions (M. Andres-Verges, et al, J. Materials Science 27 (1992) 3756-3762), by hydrothermal synthesis in solutions (Wei H. et al Materials Science and Engineering A, 393 (2005) 80-82, Bai F. et al Materials Letters 59 (2005) 1687-1690, Guo M. et al Applied Surface Science, In Press, Corrected Proof, Available online 7 Jan. 2005, Kiwamu Sue et al Materials Letters, 58 (2004) 3350-3352), by chemical bath deposition (A. M. Peiro et al Thin Solid Films, In Press, Corrected Proof, Available online 20 Jan. 2005, Zhuo Wang Journal of Solid State Chemistry, 177 (2004) 2144-2149, etc.), thermal or physical vapour deposition (Mardilovich P. et al U.S. Pat. No. 6,770,353 B1; D. W. Zeng et al, Journal of Crystal Growth, 266 (2004) 511-518), chemical vapour deposition (G. Z. Wang et al. Materials Letters, 58 (2004) 2195-2198, Jae Young Park et al, Journal of Crystal Growth, In Press, Corrected Proof, Available online 15 Dec. 2004, U.S. Patent Applications No. 2003/0213428A1 to X. Lu et al, Nos. US2004/0127130A1 and 2004/0252737A1, and PCT application WO 2004/114422A1 to Yi G. C. et al).
However, the background art does not suggest that chemical spray pyrolysis can be used for preparing highly structured zinc oxide, namely nanostructured layers comprising ZnO nanorods or nanoneedles, on various substrates.